1. Field of the Invention
This invention relates to electrical-acoustical transducers. The invention relates more particularly to an improved means for providing coupling between the transducer and the atmosphere.
2. Description of the Prior Art
Elecrical-acoustical transducers are known which can alternatively be utilized to convert electrical energy to acoustical energy when electrically excited or to convert acoustical energy to electrical energy when acoustically excited. One such form of transducer comprises a piezoelectric crystal and a vibratory surface or diaphragm which is mechanically coupled to the crystal for movement therewith. In an electrical-to-acoustical mode of operation, an exciting electrical input signal (E.sub.i) is applied to terminals of the transducer; the crystal is stressed at the frequecny (f.sub.c) of the input signal; and the vibratory surface is caused to vibrate with the stressed crystal at the frequency (f.sub.c). In an acoustical-to-electrical mode of operation, acoustical energy which is incident upon the vibratory surface causes the surface to vibrate at an exciting acoustical frequency (f.sub.c); the crystal is mechanically stressed as a result of the mechanical coupling to the vibratory surface; and, an electrical output signal (E.sub.o) of frequency (f.sub.c) is generated at the terminals of the transducer.
This form of dual operating transducer is useful in various applications where, for example, it is desirable to project ultrasonic acoustical energy and to sense reflections of the same. In these applications, which generally occur at a single frequency or at a sharply limited band of frequencies, a directional characteristic can be imparted to the transducer and the efficiency of coupling the transducer to the atmosphere can be enhanced through the use of an acoustical horn. One such arrangement utilizes a conically-shaped horm in conjunction with the transducer to project a relatively narrow beam of ultrasonic energy and to sense reflections of the same.
The above described transducer includes a vibratory surface having a finite area of predetermined configuration which is preferably symmetrical and circular. It has been found that electrical excitation of the transducer causes acoustical air pressures to be produced over a relatively limited central area of the vibratory surface which are greater than the pressures produced at segments of the vibratory surface distant from the central segment. Similarly, a concentration of exciting acoustical energy near this area produces an electrical output signal of relatively larger amplitude than is provided when the same acoustical energy is dispersed over the entire vibratory surface. This characteristic has an important effect in determining both the efficiency and sensitivity of the transducer.
It would be advantageous with respect to the efficiency of projection and with respect to the sensitivity of reception to provide an acoustical coupling means which couples the transducer to the atmosphere and which also compensates for the aforementioned characteristics of the vibratory surface.